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bits 64
section .data
usage: db 'usage: ', 0x00
params: db ' [string]', 0x00
newline: db 0x0a, 0x00
section .text
global _start
_start:
pop rax ; argc is on stack
cmp rax,1 ; see if is 1 (no parameters)
je help
pop rsi ; program name
pop rsi ; first parameter
call print
mov rsi, newline
call print
jmp exit
help:
mov rsi, usage
call print
pop rsi
call print
mov rsi, params
call print
mov rsi, newline
call print
jmp exit
print:
mov rdi, rsi ; copy, scasb uses edi
xor al, al ; al = 0
mov rcx, 4096 ; maximum length 4096
; this is used by REPNE
; and not specific to SCASB
cld
repne scasb ; scan for '\0'
; edi is now address of '\0'
sub rdi, rsi ; rdi is now length of string
mov rdx, rdi ; rdx : write(..., strlen)
sub rdx, 1 ; skip '\0'
mov rax, 1 ; syscall write()
mov rdi, 1 ; rdi : write(fd, ...)
syscall
ret
exit:
mov rax, 60 ; exit()
mov rdi, 0
syscall
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